Successful growth of epilayers on InP single crystal substrates requires careful cleaning procedures to remove surface contaminants, oxides and mechanical damage. The specific procedures vary with the investigator and the epitaxial growth techniques; however, the substrate preparation steps typically include (1) chemo-mechanical polishing of the substrate surface to obtain a flat, damage-free growth interface, (2) chemical etch of interface oxides immediately prior to growth sometimes including removal of some depth of the InP to eliminate residual mechanical damage, and (3) an in situ etch of the surface in the growth chamber to provide a freshly exposed surface in the controlled growth environment. In the chloride and hydride vapor-phase growth techniques, surface etch conditions can be provided by the growth vapors before layer growth conditions are established. However, in metalorganic vapor-phase epitaxy (MOVPE) an etch-mode cannot occur from the growth constituents so a separate etchant species must be introduced.
The use of anyhdrous HCl has been reported in two relatively recent articles by M. Razeghi, M. A. Poisson, J. P. Larivain and J. P. Duchemin, in their article in Journal of Electronic Materials 12 (1983) page 371 and J. S. Whiteley and S. K. Ghandhi in their article appearing in Journal of Electro Chemistry Society 129 (1982) 383. This use of anyhdrous HCl is a necessary etchant step for successful MOVPE preparation of the alloy semiconductor InGaAs on InP. With in situ etching of the InP substrate it is possible to achieve specular layer surfaces. HCl however is a very difficult vapor to handle. The slightest traces of moisture accelerate corrosion of metal gas regulators, flow controllers, valves and tubing. Elastomer O-ring seals are particularly vulnerable to the cross-diffusion of HCl in the delivery lines with the water vapor outside. Frequently, the lifetime of HCl gas-controlled components is short. A hazard represented by the HCl is its 630 psig cylinder pressure and potential dispersion of the corrosive toxic gas into the environment should its pressure regulator fail.
A good alternative to etchants stored under high pressure is to pass a carrier gas through an etchant liquid whose vapor pressure is sufficient to provide a useful gas concentration. Vapor etching of GaAs substrates for MOVPE growth is reported with AsCl.sub.3 as the etchant. A thorough discussion of this phenomena is set forth in the article by R. Bhat and S. K. Ghandhi, in the Journal of the Electro Chemical Society 124 (1977) 1447. The AcCl.sub.3 decomposition provides a higher purity source of HCl and compressed anhydrous HCl. Decomposition of gaseous methyl bromide, CH.sub.3 Br has been reported as photochemical etchant for GaAs and InP in the article by D. J. Ehrlich, R. M. Osgood and T. F. Deutsch appearing in Applied Physics Letter 36 (1980) 698. The gaseous methyl bromide should also be useful as a thermally-activated etchant; however, it has the disadvantage of a 1.83 atmosphere vapor pressure requiring a pressurized cylinder.
Thus there exists in the state-of-the-art a continuing need for an etchant for an indium phosphide substrate in the form of a readily available liquid having a convenient vapor pressure that lends itself for use at room temperature and is not overly corrosive, in non-flammable and doesn't react with moisture in air and further lends itself to being readily controlled.